This invention relates to a data recovery scheme for use in a telemetric measurement system, and particularly for use in a telemetric thermometer system.
U.S. patent application Ser. No. 10/017,098 filed Dec. 12, 2001 (now U.S. Pat. No. 6,629,776), the entire disclosure of which is hereby incorporated by reference herein for all purposes, discloses a telemetric thermometer system that comprises a transmitter unit and a receiver unit. The transmitter unit includes a sensor device for sensing temperature and a radio transmitter that creates and transmits packets having temperature values encoded therein. The receiver unit includes a radio receiver for receiving packets transmitted by the transmitter unit and recovering the temperature values that are encoded in the packets. The transmitter unit and the receiver unit include respective clock oscillators and internal dividers for generating a common clock frequency that is the same for both units, so that time measured by clock cycles of the receiver unit passes at the same rate as time measured by clock cycles of the transmitter unit.
The transmitter unit and the receiver unit each divide future time into operating intervals of fifteen seconds and divide each operating interval into an active interval and guard interval. Each active interval is further divided into 256 transmission slots of about 47 milliseconds.
The transmitter unit and receiver unit include respective functionally-identical pseudo-random number generators (PNGs). When the transmitter unit is first manufactured, it is placed in a “sleep” mode in which its PNG is inactive. When the transmitter unit is brought into service, it is activated by the receiver unit. Both PNGs commence operation at the same time and are seeded with the same value. Accordingly, the two PNGs operate in synchronism even though they are not connected together. For each operating interval, the PNG of the transmitter unit calculates a number in the range 1–256 and the PNG of the receiver unit calculates the same number. The number calculated by the two PNGs is used to select one of the 256 transmission slots in the operating interval. The transmitter unit transmits a packet during the transmission slot and does not otherwise transmit packets. The receiver unit is in an active state for the duration of the transmission slot, and in the active state the radio receiver is on. If the radio receiver receives a valid transmission packet, it provides an output signal from which the temperature value encoded in the transmission packet can be recovered. Otherwise, the receiver unit is in an idle state, in which the radio receiver is off.
In a proposed implementation of the thermometer described in U.S. patent application Ser. No. 10/017,098, it is possible that the receiver unit will be unable to recover a temperature value for every transmission by the transmitter unit, for example due to failure of a cyclic redundancy check or noise. The receiver unit of the telemetry system may serve several transmitter units. It is possible that two transmitter units will select the same transmission slot, in which case collision of the transmissions by the respective radio transmitters may prevent the receiver from receiving a valid packet. It is desirable that loss of data should be avoided or minimized.